Pheochromocytomas are exceedingly dangerous, but uncommon, tumors that synthesize and release epinephrine and/or norepinephrine. In our continuing efforts to improve diagnostic accuracy and to develop better means of treating these patients, we have compared the three modalities commonly used for the localization of these tumors, i.e., 131 1-MIBG, CT scan, and MRI scan. We have shown that CT scan of the adrenal areas is generally the first choice in localizing these tumors because of its ready availability, low cost, and good delineation of anatomical features. We have found that MIBG scintigraphy is 100% specific and especially indicated where either multiple tumors of malignant disease is suspected. We have demonstrated in preliminary studies that measurement of plasma normetanephrine (NMN) and metanephrine (MN) is readily possible in man and may be especially useful in the diagnosis of pheochromocytoma, since plasma NMN appears to have greater sensitivity and specificity in the diagnosis of this disorder.